This invention relates to a speech encoding system for use in encoding and decoding a speech signal by the use of a regular pulse excitation technique and, in particular, to an analyzer and a synthesizer for analyzing and synthesizing the speech signal.
A conventional speech encoding system of the type described is disclosed in an article contributed by Ed. F. Depretter and Peter Kroon to ICASSP, 1985 and proposed under the title of "Regular Excitation Reduction for Effective and Efficient LP-Coding of Speech" (pages 965 to 968). The proposed system is referred to as a regular pulse excitation system and is effective to encode a waveform of the speech signal, differing from a multipulse excitation system based on a spectrum analysis of a speech signal, as proposed by Atal et al. The regular pulse excitation system comprises an analysis side (namely, an analyzer) and a synthesis side (namely, a synthesizer) for analyzing and synthesizing the speech signal, respectively. More specifically, an input speech signal is subjected to linear predictive coding (LPC) to obtain a sequence of linear predictive coding (LPC) coefficients and to represent an envelope of the input speech signal. In addition, the speech signal of an exciting source is specified in the analyzer by a sequence of impulses which are arranged at an equal time instant and which are variable in phases and amplitudes. At any rate, the impulse sequence is delivered from the analyzer to the synthesizer as a part of analyzed data signals.
With this system, it is possible to faithfully reproduce the speech signal in the synthesizer as compared with the multipulse excitation system because the waveform of the speech signal itself is reproduced in the synthesizer. As a result, a reproduced speech signal naturally sounds without any unevenness. This means that a speech quality is improved in the regular pulse excitation system in comparison with the multipulse excitation system. In other words, the regular pulse excitation system has a speech quality which is not varied in dependence on sounds of persons and which is invariable.
However, the conventional regular pulse excitation system should encode a set of the analyzed data signals at a rate which is equal to or higher than 9.6 kb/s. Accordingly, it is difficult to transmit such analyzed data signals at a low bit rate lower than 9.6 kb/s.
On the other hand, a recent requirement is to transmit the analyzed data signals at a very low bit rate, such as 2.4 kb/s, to effectively utilize a transmission path.